1. Field of the Invention
The present invention relates generally to techniques for determining functional residual capacity (FRC), the volume of gases that remain within a subject's lungs following exhalation, or, more broadly, the effective lung volume (ELV) of the subject, which includes gases that have diffused into the lung tissues. In particular, the present invention relates to techniques for noninvasively determining FRC or ELV.
2. Background of Related Art
Functional residual capacity (FRC) is the volume of gases, including carbon dioxide (CO2), that remains within the lungs of a subject at the end of exhalation, or expiration. In healthy individuals, FRC usually comprises about 40% of total lung capacity, and typically amounts to about 1.8 liters to about 3.4 liters. FRC buffers against large breath-to-breath changes in the amount of carbon dioxide in the alveoli of the subject's lungs, which may be measured in terms of partial pressure of CO2 (pACO2) or as a fraction of gases that comprise CO2 (fACO2). With normal tidal volumes, pACO2 and fACO2 typically fluctuate by only about 2 mmHg or about 0.25%, respectively.
A number of authors contend that CO2 is stored in the lungs in three different compartments: (1) the gas volume (VA or FRC); (2) the lung tissue; and (3) the pulmonary blood present at any given time in the lung. The lung tissue and pulmonary blood compartments are often represented in terms of their equivalent gas volumes (i.e., scaled by their effective storage capacity) and denoted Vtis and Vblood. While FRC only accounts for the volume of gases (including CO2) in the alveoli, effective lung volume (ELV) includes FRC, as well as gases that remain diffused within the tissues of the lungs of the subject at the end of exhalation and, therefore, accounts for gases in all three compartments.
While ELV is typically a slightly larger volume than FRC, these terms may be used interchangeably in the ensuing description for purposes of simplicity.
Each compartment equilibrates with changes in CO2 at a different rate. Gedeon, A., et al., “Pulmonary blood flow (cardiac output) and the effective lung volume determined from a short breath hold using the differential Fick method,” J. CLIN. MONIT. 17:313-321 (2002) (hereinafter “Gedeon 2002”) teaches that VA equilibrates instantly with changes in end tidal CO2 (petCO2 when measured in terms of partial pressure and fetCO2 when measured in terms of the fraction of gases that comprise CO2) and slowly (e.g., in about ten to about twenty seconds) with changes in pACO2 and content of CO2 in arterial blood (caCO2), while it takes less time for Vtis and Vblood to equilibrate when pACO2 and caCO2 change.
The relationship between a subject's chest wall and lungs and the elastic recoil of the lungs defines FRC and, thus, ELV. Lung diseases that change the elastic recoil of the lungs, including emphysema, asthma, and other restrictive diseases, affect FRC. Thus, FRC determinations may be useful in accurately diagnosing such conditions. FRC determinations are also useful in diagnosing and treating respiratory failure and hypoxemia.
In lungs with an FRC below the lung's closing capacity, the airways start to close before the end of a subject's exhalation, which results in a decrease of pAO2 and a mismatch between ventilation, or the movement of gases into and out of the lungs through the mouth, and perfusion, or the movement of gases across the gas/blood barrier between the alveoli of the lungs and the pulmonary capillaries that surround the alveoli. This is known in the art as V/Q mismatch or VT/VQ mismatch.
The currently available techniques for measuring FRC include full body plethysmography, nitrogen washout, and helium dilution. All of these methods require cumbersome equipment and, therefore, may not be suitable for use in an intensive care setting that is already crowded with equipment.
Gedeon 2002 proposed a noninvasive technique for determining ELV. Specifically, that technique includes measuring the {dot over (V)}MCO2 and fetCO2 of a subject, having the subject hold his or her breath for three seconds, the re-measuring {dot over (V)}MCO2 and fetCO2. For the first breath following the breath-hold, fetCO2 increases and {dot over (V)}MCO2, which is calculated over the duration of the breath hold and the subsequent breath, decreases. Assuming, due to buffering by the CO2 stores of the ELV, that {dot over (V)}BCO2 (i.e., CO2 passing from the pulmonary capillary blood into the alveoli of the lung) does not change during breath-holding, Gedeon contends that the decrease in {dot over (V)}MCO2 must have resulted from the CO2 going into the lung stores of CO2:[fetCO2 POST−fetCO2 PRE]×{circumflex over (V)}A*=[{dot over (V)}MCO2PRE−{dot over (V)}MCO2POST]×[Tbreath+Tbreathhold]where [{dot over (V)}MCO2PRE and {dot over (V)}MCO2POST refer to measurements obtained respectively before and after the breath-holding maneuver.
In addition to this relationship, Gedeon developed equations that relate pulmonary capillary blood flow (PCBF or, for the sake of simplicity in the ensuing equations, {dot over (Q)}) of the subject to the subject's ELV. Two of these equations compare the pre-breathhold conditions to the post-breathhold conditions and the pre-breathhold conditions to the recovery conditions. The ELV of an ELV and PCBF data pair that satisfies both of these equations is considered to be the subject's actual ELV.
The technique of Gedeon 2002 is believed to provide inaccurate data, as it is based on the assumption that “CO2 inflow [may] not [be] significantly affected” by breath-holding, while breath-holding will cause a change in pACO2. This assumption is inconsistent with the Fick equation, in which {dot over (V)}BCO2 changes linearly with pACO2 while PCBF and the amount of CO2 in the venous blood (cvCO2 or, as Gedeon2002 refers to it, pven) remain constant.
In view of the foregoing, it is apparent that there is a need for a technique for accurately, noninvasively measuring FRC or ELV in virtually any healthcare setting.